Issue description

Using the new Postgresql design when extensions are added to Postgresql using extraPlugins a new postgresql-and-plugins derivation is created.

While this solves a lot of other issues, one it creates is that adding extensions causes the Postgresql service to be restarted (which isn't strictly needed).

Steps to reproduce

Start a postgresql service, add

extraPlugins = [ pg_hll ] 

to the config and switch to it - a new postgresql-and-plugins derivation will be created and activated, which will cause a service stop / start.

Technical details

• system: "x86_64-linux"
• host os: Linux 4.19.37, NixOS, 19.09pre179307.bc94dcf5002 (Loris)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.2.2
• channels(jirotech): ""
• channels(root): "nixos-19.09pre179307.bc94dcf5002"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos